1. Field of the Invention
The present invention relates to a wireless mobile communication system, and more particularly, to a method of transmitting data to mobile terminals in the wireless mobile communication system capable of transmitting multicast data.
2. Description of the Related Art
The present invention is directed to a communication method applicable to a mobile communication system capable of transmitting multicast data. Hereinafter, a communication system using a wireless local area network (LAN) interface will be described as an example of the mobile communication system capable of transmitting multicast data.
FIG. 1 is a view showing the structure of a frame used in a wireless LAN (IEEE 802.11).
FIG. 2 is a view showing an example of a management frame used in a wireless LAN. As shown in the figure, an example of the management frame can be a beacon, a probe request, and a probe response.
FIG. 3 is a view showing management frame body components used in the IEEE 802.11. As shown in the figure, the management frame body comprises a fixed field area and an information element area.
FIG. 4 is a view showing the structure of the Internet group management protocol (IGMP) (version 2) necessary for a multicast service. The IGMP is an Internet protocol that provides a means for allowing an Internet computer to inform neighboring routers of a multicast group. The multicast service is performed in an upper layer (e.g., an IP layer), and a wireless LAN interface relays the multicast service.
Hereinafter, a general configuration of a wireless LAN (IEEE 802.11) system will be described.
The wireless LAN is a network environment for providing LAN services to wireless terminals equipped with wireless LAN cards, such as a personal data assistant (PDA) or a notebook personal computer (PC), through an access point (AP) corresponding to a hub of a wired LAN. In other words, it can be considered as a system in which a wireless section between the AP and a network interface card (NIC) such as a wireless LAN card is substituted for a wired section between a hub and a user terminal in an existing Ethernet system. The wireless LAN has advantages in that a wireless terminal can be easily relocated and a network can be easily constructed and expanded since no wiring is required. It is also possible to communicate while moving. However, there are disadvantages in that the transmission speed is relatively low compared with a wired LAN, and signal quality is unstable and signal interference can occur due to characteristics of a wireless channel.
FIG. 5 is a view showing the configuration of a network based on a wireless LAN interface. As shown in the figure, wireless LAN networks are classified into two types according to whether a network includes an AP. A network which includes an AP is called an infrastructure network, and a network which does not include an AP is called an ad-hoc network. A service area provided by a single AP is called a basic service area (BSA), and an AP and wireless terminals connected thereto are called a basic service set (BSS). A service which a wireless terminal connected to the AP receives in such a manner is called a station service (SS). The SS also includes services exchanged between wireless terminals in the ad-hoc network. As shown in FIG. 5, the BSAs may overlap one another. Two or more APs can be interconnected with one another so that a wireless terminal connected to one AP can communicate with other wireless terminals connected to the other APs. In this case, the connection between the APs is called a distribution system (DS), and a service provided through the DS is called a distribution system service (DSS). In addition, an area where the DSS can be provided is called an extended service area (ESA), and all wireless terminals and APs that receive the DSS within the ESA are called an extended service set (ESS).
There are nine service items defined in the IEEE 802.11 standard as shown in Table 1.
TABLE 1a)Authenticationb)Associationc)Deauthenticationd)Deassociatione)Distributionf)Integrationg)Privacyh)Reassociationi)MSDU delivery
Authentication and deauthentication are related to the user authentication; and association, deassociation, and reassociation are related to the connection of a wireless terminal to an AP. Reassociation is used when a wireless terminal changes the BSS within the ESS or when a current connection state is changed. A distribution service is a conceptual service by which a wireless terminal connected to an AP through the DS can communicate with a wireless terminal connected to another AP. Integration is used for connecting an IEEE 802.11 LAN to an external general LAN that may be a wired or wireless LAN.
These items can be modified through DS settings. Privacy is related to security and uses the wired equivalent privacy (WEP) protocol. MAC service data unit (MSDU) delivery is used to transmit user data. These service items are grouped and divided into two service categories described above, which are shown in Table 2.
TABLE 2SS (Station Service)DSS (Distribution System Service)a) Authenticationa) Associationb) Deauthenticationb) Deassociationc) Privacyc) Distributiond) MSDU deliveryd) Integratione) Reassociation
A wireless LAN AP performs a variety of functions similar to functions of a hub of a wired LAN, including a bridge function, home gateway function, automatic fallback function, roaming function and the like. The bridge function enables communications between two buildings spaced apart from each other using a directional high-gain external antenna. The home gateway function allows home information communication equipment to be connected to an external network through the wireless LAN AP. The automatic fallback function is a function of lowering the transmission speed of the AP from 11 Mbps to 5.5 Mbps, 2 Mbps or 1 Mbps when a distance between the AP and a wireless terminal is increased and thus the channel state becomes worse. If a high speed transmission of 11 Mbps is maintained even though the channel state is not good, losses due to retransmission or the like are greatly increased, and thus, the transmission speed is properly lowered. The basic roaming function is permitted between the BSSs.
Hereinafter, the operation of receiving a multicast service in a conventional wireless network is described. When a mobile terminal (STA: station) receives a multicast service from a network through a wireless network based on a wireless LAN interface, a procedure shown in FIG. 6 will be performed.
As shown in FIG. 6, a multicast router informs all the connected APs of information on the multicast service through an IGMP query (S601). The AP transmits the IGMP query through a broadcast message again, and thus, the mobile terminals can receive the relevant message (S602). If each of the mobile terminals wishes to receive the multicast service, it joins the relevant multicast service by transmitting an IGMP report (S603). As shown in the figure, the IGMP report is transferred to the AP which in turn transfers the IGMP report to the multicast router.
The multicast router transmits multicast data to the locations where relevant terminals are placed (S604). That is, the AP transmits the multicast data received from the multicast router to each terminal, so that the multicast service can be performed (S605).
The multicast data may be transmitted at specific intervals. If there are no multicast data to receive according to the multicast service interval, a mobile terminal can enter a power saving (PS) mode (S606).
All the mobile terminals should be ready to receive the IGMP query that is periodically transmitted by the multicast router. To this end, all the terminals should wake up at a specific time point (S607). The multicast router transmits an IGMP query to all the connected APs at a specific interval, e.g. every query interval (S608). The AP transmits the IGMP query to the terminals in the same manner as step S602 (S609). The IGMP report is periodically transmitted if a terminal desires to continue receiving the relevant multicast service (S610). In FIG. 6, a first terminal STA#1, a third terminal STA#3, and a fourth terminal STA#4 can know that a second terminal STA#2 transmits an IGMP report to the AP. Accordingly, the first, third and fourth terminals STA#1, STA#3, and STA#4 do not transmits an IGMP report (S611 and S612). All the terminals can enter the PS mode again according to the multicast service interval (S613).
At the end of each multicast service interval, the relevant terminals should wake up to receive the multicast service again (S614).
The multicast router transmits multicast data to the AP where the relevant terminals are connected (S615). The AP transmits the multicast data received from the multicast router to each terminal, so that the multicast service can be performed (S616).
Hereinafter, a case where a mobile terminal which has received a multicast service from a specific first AP moves into a second AP will be explained.
FIG. 7 is a flowchart illustrating the operation that is performed when a mobile terminal moves from a specific network into another network while receiving a multicast service.
In FIG. 7, the multicast router informs all the connected APs AP1 and AP2 of information on the multicast service for the terminals using an IGMP query (S701). The APs AP1 and the AP2 transmit the IGMP query through a broadcast message, and the mobile terminals can receive the relevant message (S702). If each of the mobile terminals wishes to receive the multicast service, it joins the relevant multicast service by transmitting an IGMP report (S703, S704 and S705).
The multicast router receives the IGMP reports and transmits multicast data to the locations where the relevant terminals are placed (S706). The APs AP1 and the AP2 transmit the multicast data received from the multicast router to the respective terminals, so that the multicast service can be provided (S707).
Until step S707, the first terminal STA#1 is connected to the AP AP1 and the second and third terminals STA#2 and STA#3 are connected to the AP AP2. Hereinafter, a case where the first terminal moves into an area of the AP AP2 is described. The first terminal STA#1 belonging to the AP1 moves into the area of the AP AP2 (S708).
New multicast data are transmitted to all the APs (S709). The APs AP1 and the AP2 transmit the received multicast data to the respective terminals. That is, although there is no mobile terminal belonging to the AP AP1, the AP AP1 allocates wireless resources and transmits the multicast data (S710). The AP AP2 transmits the multicast data to relevant terminals within the network (S711).
Hereinafter, problems of the prior art will be explained.
When an IGMP query is transmitted to each of terminals according to the prior art, all of mobile terminals that wish to receive the multicast service should be ready to receive the IGMP query. That is, all the mobile terminals should be kept at a state where data can be received. However, a series of procedures for maintaining an activation state to receive the IGMP report query are performed regardless of a series of procedures for saving power of a terminal.
Due to the characteristics of the prior art, there is a certain limitation in maximizing the effect of saving electric power.
Further, due to the above characteristics, in the worst case, there may be an occasion where all the terminals enter the PS mode and no terminal transmits an IGMP report. That is, there may be a problem in that all the terminals cannot receive the multicast service.
When multicast data are transmitted according to the prior art and a terminal that wishes to receive a multicast service moves into an area of a new AP, information about the movement of the terminal is not transferred to the previous AP, and thus, the previous AP should continue distributing the multicast data over the network. Due to the above characteristics, there is another problem in that wireless resources cannot be efficiently used.
In addition, when multicast data are transmitted according to the prior art, a method of transmitting the data is fixed to a point-to-point bearer mode or a point-to-multipoint bearer mode. Therefore, there is a further problem in that an AP cannot receive a sufficient amount of feedback information while freely transmitting data without regard to the number of terminals.